This invention relates generally to fluidized bed hearths designed for burning very finely divided solid fuels such as for example flotation wastes or separation sludges, and in particular to a method of and a device for feeding such solid fuels into the fluidized bed hearth.
In solid fuel fired hearths of this type which use the very finely divided fuel material, feeding problems frequently occur which impede the proper operation of the hearth. Normally, it is desired that in order to achieve a simple operation of the combustion process proper and an improved binding of undesired component parts such as sulfur from the exhaust gas into the slag or ashes it is desirable to feed a moisture-free solid fuel such as coal, for example.
The dry solid fuel, however, cannot be fed on the fluidized bed simply from above since in this case it would burn only incompletely and would have to be returned into the combustion fluidized bed for additional burning. As a consequence, a high recycling ratio of the fed fuel would result.
Conventional mechanical or pneumatical feeding of the fuel into the lower portion of one side of the combustion part of the fluidized bed is applicable for small fluidized bed hearths only. In the case of large sized fluidized beds the mixing effect in horizontal plane is too small for achieving a satisfactory combustion of the fed-in fuel.
It is true that it is possible to feed the finely divided fuel via nozzles similarly as it is known in the case of liquid or gaseous fuels, nonetheless due to the necessary distribution of the fuel an excessive number of such nozzles would have to be employed and this solution therefore would become too expensive. In addition, due to the high contents of flyash in the recycled fuel returned by pneumatic conveying means which is to be provided with a plurality of branches with distributors and elbow pipes, a considerable wear of the conveying system would result.
Some of the known fuel feeding devices for the fluidized bed hearths are described in an article "Dampferzeuger mit Wirbelschichtfeuerung unter atmospharischen und Uberdruckbedingungen" in a printed copy 41/5/78/2000/d of the firm Vereinigten Kesselwerke Ag, Dusseldorf, Federal Republic of Germany.